The present invention relates to composite structures, and more particularly to composite structures comprising a matrix phase and a reinforcement phase, wherein the matrix phase is a sintered ceramic or metal and the reinforcement phase consists of elongated ceramic reinforcing elements of sheet or tape configuration which are co-sintered with the matrix to provide strengthened and toughened composite structures.
Ceramic matrix composites comprise a unique subgroup of reinforced composite materials. Such composites typically comprise a glass, glass-ceramic, or ceramic material forming a continuous matrix phase within which are disposed a multiplicity of inorganic reinforcing whiskers or fibers. The presence of fibers strengthens the ceramic matrix material and also imparts a degree of toughness thereto such that, instead of exhibiting sudden, brittle and catastrophic breakage, the composite material fails in a more "graceful" (gradual) manner. This desirable failure mode results from the presence of the fibers which tend to bridge faults in the matrix material and retard crack enlargement by the mechanism of relatively slow and energy dissipating "fiber pullout".
Representative of ceramic matrix composites for high stress, high temperature applications are the fiber reinforced glass-ceramic matrix composites containing silicon carbide fiber reinforcement- These are widely described in the patent literature; representative patents describing such composites include U.S. Pat. Nos. 4,626,515 and 4,615,987.
One economic disadvantage associated with fiber-reinforced composites such as described in these patents relates to the need for hot-pressing at high temperatures and pressures to produce truly dense and void-free products. Difficulties are encountered in attempting to consolidate ceramic matrix materials around rigid fibers and/or whiskers, presumably because the fibers and whiskers oppose the relatively weak forces of sintering and impede the natural shrinkage of the matrix.
A further disadvantage is that high temperature hot-pressing is not well suited to the fabrication of complex structures. Thus products resulting from such processes are significantly restricted as to size and shape. In addition, hot pressing requires costly mold sets, and often a chemically reducing process environment which requires special equipment to maintain.
The numbers and types of fiber and whisker compositions available to the art for use in fiber composite systems are also somewhat limited. This factor limits the range of composition for compatible matrix materials, which in turn limits the options available to the composite designer for "engineering" the fiber/matrix interface. Control over the composition and properties of this interface is a key element in the development of composite systems offering enhanced "fiber pullout" or work-of-fracture behavior.
One method for the fabrication of fiber-reinforced composites while avoiding hot pressing, described in U.S. Pat. No. 5,053,175, is to combine unsintered ceramic fibers with unsintered ceramic matrix sheets by cold-pressing, and then to consolidate the fibers and matrix together by heating. However, this approach can suffer from difficulties relating to inadequate fiber pullout behavior and/or weakness in the sintered fibers due to consolidation defects.
Yet another approach to the fabrication of tough ceramics involves the development of laminated ceramics comprising weak interlayers or "zones of weakness" in the laminate. Laminar ceramics of this type have been described by D. B. Marshall et. al. in "Enhanced Fracture Toughness in Layered Microcomposites of Ce--ZrO.sub.2 and Al203", J. Am. Ceram Soc., 74 [12] pages 2979-2987 (1991), and in published European patent application No. EP 0 441 528. These structures, however, are somewhat limited in flexural strength and transverse flexural strength, due to the strong tendency of the products to delaminate at the interlayers.
It is therefore a principal object of the present invention to provide ceramic or metal matrix composite products over a broad range of matrix and reinforcement composition offering both improved strength and improved work of fracture performance.
It is a further object of the present invention to provide new types of reinforced composites which can be fabricated without the need for high temperature hot-press consolidation.
It is a further object of the invention to provide new reinforced ceramic matrix composites with enhanced fracture toughness which also offer high resistance to delamination under flexural stress.
It is a further object of the invention to provide an improved method for the fabrication of ceramic or metal matrix composites which avoids hot pressing while at the same time offering more flexibility in composite design.
Other objects and advantages of the invention will become apparent from the following description thereof.